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Plague is a rare, life-threatening, flea-borne zoonosis caused by the bacterium Yersinia pestis, which is a gram-negative coccobacillus. The infection is primarily spread through rodent-associated fleas. Transmission to humans occurs through the bite of infected fleas, direct contact with infected body fluids or tissues, or inhalation of respiratory droplets from ill persons or animals, including domesticated cats and dogs. The usual incubation period between exposure and illness onset is 2 to 7 days. Human to human transmission is possible in cases of pneumonic plague.

A recent article in Emerging Infectious Diseases reviewed cases of plague in the United States between the years of 1970 and 2017. During this time, 482 human plague cases were reported, of which 258 had an identifiable animal exposure. Most of the animal exposures involved domestic animals that were infected either through predation of rodents or flea infestations. Veterinarians and veterinary technicians were at highest risk of occupational exposure. The states of New Mexico, Colorado, and Arizona had the highest number of reported human plague cases.

Plague infection is characterized by the sudden onset of fever and malaise, which may be accompanied by abdominal pain, nausea, and vomiting. There are three main forms of plague depending upon the route of infection. Bubonic plague is caused by the bite of an infected flea and accounts for approximately 80% to 85% of cases. Patients develop a bubo, which is a painful swelling of one or more lymph nodes during the first few days of illness. Septicemic plague occurs from a flea bite or direct contact with infectious fluids and accounts for approximately 10% of cases. Infection spreads directly through the bloodstream with no local signs. Primary pneumonic plague results from aerosol exposure to infective droplets. It is characterized by fulminant primary pneumonia and accounts for approximately 3% of cases. Secondary pneumonic plague can result from the spread of Y. pestis to the lungs in patients with untreated bubonic or septicemic infection.

The mortality rate for untreated plague is approximately 50% in cases of bubonic plague and near 100% in septicemic or pneumonic plague. Prompt treatment with antimicrobials such as aminoglycosides, fluoroquinolones, or doxycycline has reduced the mortality rate to approximately 16%.

Depending on clinical presentation, appropriate specimens should be obtained for Gram stain and bacterial culture:

  • Lymph node aspirate from a bubo
  • Blood cultures:
  • Sputum
  • Bronchial or tracheal washing

Automated bacterial identification systems have been reported to misidentify Y. pestis as Pseudomonas luteola.

If cultures yield negative results, and plague is still suspected, serologic testing may help to confirm the diagnosis. One serum specimen should be taken as early in the illness as possible, followed by a convalescent sample 4 to 6 weeks or more after disease onset.

Cases of suspected or confirmed plague should be immediately reported to public health authorities.

References, December 2019, 25:12;2270-2272

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